Breast-feeding is the predominant transmission route for HIV-1 infection in children. However, the vast majority of breast-fed infants do not become HIV- infected despite continuous exposure to the virus through their mothers' milk over many months. What protects breast-fed infants from HIV-1 infection? HIV-1 entry across the infant's mucosal barrier is partially mediated through binding of the viral envelope glycoprotein gp120 to the glycan binding domain of DC-SIGN on mucosal dendritic cells. DC-SIGN not only binds to the glycans on gp120, but has an even higher binding affinity for the Lewis antigens, which are also glycans and compete with gp120 for DC-SIGN binding. One liter of human milk contains 5-10 g unbound oligosaccharides, complex glycans, which carry Lewis blood group antigens. We hypothesize that human milk oligosaccharides (HMO) compete with HIV-1-gp120 for DC-SIGN binding, reduce HIV-1-transmission through breast milk, and protect a majority of infants from HIV-1-infection. Our Preliminary Results in two independent in vitro assays show that HMO indeed reduce gp120-binding to DC-SIGN by more than 80%. However, HMO consist of more than 150 different oligosaccharides. Which ones are most effective? AIM1 uses Frontal-Affinity-Chromatography Mass Spectrometry to identify individual HMO with high binding affinity for DC-SIGN. AIM2 then uses the two already established in vitro assays to assess whether the identified HMO inhibit HIV-1- gp120 binding to DC-SIGN. The results will guide in vitro and in vivo studies investigating whether the identified oligosaccharides reduce HIV-1 infection. Other viruses such as Hepatitis C, Ebola or Dengue, also use DC-SIGN as their entry point. The identified milk oligosaccharides could serve as drug design temples to fight these devastating diseases. [unreadable] [unreadable] PUBLIC HEALTH RELEVANCE: Although breast-feeding transmits the HIV-1 virus from mothers to their infants, breast milk contains compounds that protect a majority of infants from infection. Preliminary Results support our hypothesis that human milk oligosaccharides are part of this protective mechanism. We aim at identifying those individual human milk oligosaccharides that are most effective in reducing HIV-1 infection. [unreadable] [unreadable] [unreadable] [unreadable]